Natural rubber has hitherto been used widely as industrial products, such as automobile tires, belts and pressure-sensitive adhesives, and household goods, such as gloves. These natural rubber articles are generally produced by coagulating the rubber content of a natural rubber latex to obtain raw rubber called crepe rubber or smoked sheet rubber, and further processing the raw rubber through steps of mastication, compounding of additives, molding, and vulcanization.
It was recently reported that medical tools made of natural rubber, such as surgical gloves, various catheters, and analgesic masks, provoke labored respiration or anaphylactoid symptoms, such as vascular edema, nettle rash, detelectasis and cyanosis, in patients. Cases were also reported in which women with a history of allergy suffered a pain in the hands, nettle rash or vascular edema around the eyes when they used rubber gloves made of natural rubber.
These symptoms seem to be attributed to the protein present in natural rubber. Food and Drug Administration (FDA), U.S.A. has called on manufactures of natural rubber articles to reduce the protein content. It has therefore been demanded to remove protein from natural rubber.
Natural rubber is obtained from Hevea trees as a latex containing a rubber content, water, protein, inorganic salts, and other impurities. The latex oozing out from the tapped trunk of a rubber plant is collected in a cup, gathered at a refining factory where it is coagulated to obtain raw rubber (crepe rubber or smoked sheet rubber) or concentrated by centrifugation to obtain a purified latex.
The protein content in natural rubber has usually been expressed in terms of a nitrogen content (N %) determined by a Kjeldahl method multiplied by 6.3. The present inventors discovered that the proteins in raw rubber obtained from a latex can be confirmed by infrared absorption at 3280 cm.sup.-1 characteristic of polypeptide.
The present inventors previously found that a deproteinized natural rubber latex showing no IR absorption at 3280 cm.sup.-1 can be obtained by a process comprising treating a natural rubber latex with a protease and a surfactant either simultaneously or successively and, after allowing the system to stand for a given period of time, recovering the rubber content by centrifugation (see Japanese Patent Application Nos. 208754 to 208758/92 (corresponding to EP-A-0 584 597)).
General natural rubber latex on the market are stable (not coagulated) against mechanical stimulation, such as vigorous stirring or friction. Such stability of latex is measured as mechanical stability (JIS K-6381).
The above-mentioned deproteinized latex has a disadvantage in that the mechanical stability is seriously poorer than general latex on the market. While the reason for the serious reduction in mechanical stability has not yet been clarified, it is considered that the protein contained in general latex is adsorbed by rubber particles and assumes strong negative charges under a condition made alkaline by ammonia addition so that the rubber particles are present in the latex in a considerably stabilized state. To the contrary, not containing such protein acting as a protective layer, the deproteinized latex seems to be labile and apt to undergo agglomeration on receipt of mechanical stimulation.
Additionally, the surfactant which has been used for protein removal is almost removed by subsequent centrifugation and scarcely remains in the deproteinized latex.